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<div class="title">ring_queue_lockfree.h</div>  </div>
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<div class="fragment"><div class="line"><a name="l00001"></a><span class="lineno">    1</span>&#160;<span class="preprocessor">#pragma once</span></div>
<div class="line"><a name="l00002"></a><span class="lineno">    2</span>&#160; </div>
<div class="line"><a name="l00003"></a><span class="lineno">    3</span>&#160;<span class="keyword">namespace </span>resumef</div>
<div class="line"><a name="l00004"></a><span class="lineno">    4</span>&#160;{</div>
<div class="line"><a name="l00005"></a><span class="lineno">    5</span>&#160;    <span class="comment">//目前无法解决三个索引数值回绕导致的问题</span></div>
<div class="line"><a name="l00006"></a><span class="lineno">    6</span>&#160;    <span class="comment">//如果为了避免索引回绕的问题，索引采用uint64_t类型，</span></div>
<div class="line"><a name="l00007"></a><span class="lineno">    7</span>&#160;    <span class="comment">//则在与spinlock&lt;T, false, uint32_t&gt;版本的对比中速度反而慢了</span></div>
<div class="line"><a name="l00008"></a><span class="lineno">    8</span>&#160;    <span class="comment">//pop时无法使用move语义来获取数据。因为算法要求先获取值，且获取后有可能失败，从而重新获取其它值。</span></div>
<div class="line"><a name="l00009"></a><span class="lineno">    9</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty = u<span class="keywordtype">int</span>32_t&gt;</div>
<div class="line"><a name="l00010"></a><span class="lineno">   10</span>&#160;    <span class="keyword">struct </span>ring_queue_lockfree</div>
<div class="line"><a name="l00011"></a><span class="lineno">   11</span>&#160;    {</div>
<div class="line"><a name="l00012"></a><span class="lineno">   12</span>&#160;        <span class="keyword">using</span> value_type = _Ty;</div>
<div class="line"><a name="l00013"></a><span class="lineno">   13</span>&#160;        <span class="keyword">using</span> size_type = _Sty;</div>
<div class="line"><a name="l00014"></a><span class="lineno">   14</span>&#160;    <span class="keyword">public</span>:</div>
<div class="line"><a name="l00015"></a><span class="lineno">   15</span>&#160;        ring_queue_lockfree(<span class="keywordtype">size_t</span> sz);</div>
<div class="line"><a name="l00016"></a><span class="lineno">   16</span>&#160; </div>
<div class="line"><a name="l00017"></a><span class="lineno">   17</span>&#160;        ring_queue_lockfree(<span class="keyword">const</span> ring_queue_lockfree&amp;) = <span class="keyword">delete</span>;</div>
<div class="line"><a name="l00018"></a><span class="lineno">   18</span>&#160;        ring_queue_lockfree(ring_queue_lockfree&amp;&amp;) = <span class="keywordflow">default</span>;</div>
<div class="line"><a name="l00019"></a><span class="lineno">   19</span>&#160;        ring_queue_lockfree&amp; operator =(<span class="keyword">const</span> ring_queue_lockfree&amp;) = <span class="keyword">delete</span>;</div>
<div class="line"><a name="l00020"></a><span class="lineno">   20</span>&#160;        ring_queue_lockfree&amp; operator =(ring_queue_lockfree&amp;&amp;) = <span class="keywordflow">default</span>;</div>
<div class="line"><a name="l00021"></a><span class="lineno">   21</span>&#160; </div>
<div class="line"><a name="l00022"></a><span class="lineno">   22</span>&#160;        <span class="keyword">auto</span> size() const noexcept-&gt;size_type;</div>
<div class="line"><a name="l00023"></a><span class="lineno">   23</span>&#160;        auto capacity() const noexcept-&gt;size_type;</div>
<div class="line"><a name="l00024"></a><span class="lineno">   24</span>&#160;        <span class="keywordtype">bool</span> empty() const noexcept;</div>
<div class="line"><a name="l00025"></a><span class="lineno">   25</span>&#160;        <span class="keywordtype">bool</span> full() const noexcept;</div>
<div class="line"><a name="l00026"></a><span class="lineno">   26</span>&#160;        template&lt;class U&gt;</div>
<div class="line"><a name="l00027"></a><span class="lineno">   27</span>&#160;        <span class="keywordtype">bool</span> try_push(U&amp;&amp; value) noexcept(std::is_nothrow_move_constructible_v&lt;U&gt;);</div>
<div class="line"><a name="l00028"></a><span class="lineno">   28</span>&#160;        <span class="keywordtype">bool</span> try_pop(value_type&amp; value) noexcept(std::is_nothrow_move_constructible_v&lt;value_type&gt;);</div>
<div class="line"><a name="l00029"></a><span class="lineno">   29</span>&#160;    private:</div>
<div class="line"><a name="l00030"></a><span class="lineno">   30</span>&#160;        std::unique_ptr&lt;value_type[]&gt; m_bufferPtr;</div>
<div class="line"><a name="l00031"></a><span class="lineno">   31</span>&#160;        size_type m_bufferSize;</div>
<div class="line"><a name="l00032"></a><span class="lineno">   32</span>&#160; </div>
<div class="line"><a name="l00033"></a><span class="lineno">   33</span>&#160;        std::atomic&lt;size_type&gt; m_writeIndex;        <span class="comment">//Where a new element will be inserted to.</span></div>
<div class="line"><a name="l00034"></a><span class="lineno">   34</span>&#160;        std::atomic&lt;size_type&gt; m_readIndex;         <span class="comment">//Where the next element where be extracted from.</span></div>
<div class="line"><a name="l00035"></a><span class="lineno">   35</span>&#160;        std::atomic&lt;size_type&gt; m_maximumReadIndex;  <span class="comment">//It points to the place where the latest &quot;commited&quot; data has been inserted. </span></div>
<div class="line"><a name="l00036"></a><span class="lineno">   36</span>&#160;                                                    <span class="comment">//If it&#39;s not the same as writeIndex it means there are writes pending to be &quot;commited&quot; to the queue, </span></div>
<div class="line"><a name="l00037"></a><span class="lineno">   37</span>&#160;                                                    <span class="comment">//that means that the place for the data was reserved (the index in the array) </span></div>
<div class="line"><a name="l00038"></a><span class="lineno">   38</span>&#160;                                                    <span class="comment">//but the data is still not in the queue, </span></div>
<div class="line"><a name="l00039"></a><span class="lineno">   39</span>&#160;                                                    <span class="comment">//so the thread trying to read will have to wait for those other threads to </span></div>
<div class="line"><a name="l00040"></a><span class="lineno">   40</span>&#160;                                                    <span class="comment">//save the data into the queue.</span></div>
<div class="line"><a name="l00041"></a><span class="lineno">   41</span>&#160;    <span class="preprocessor">#ifdef _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00042"></a><span class="lineno">   42</span>&#160;        std::atomic&lt;size_type&gt; m_count;</div>
<div class="line"><a name="l00043"></a><span class="lineno">   43</span>&#160;<span class="preprocessor">    #endif</span></div>
<div class="line"><a name="l00044"></a><span class="lineno">   44</span>&#160; </div>
<div class="line"><a name="l00045"></a><span class="lineno">   45</span>&#160;        <span class="keyword">auto</span> countToIndex(size_type a_count) <span class="keyword">const</span> noexcept-&gt;size_type;</div>
<div class="line"><a name="l00046"></a><span class="lineno">   46</span>&#160;        <span class="keyword">auto</span> nextIndex(size_type a_count) <span class="keyword">const</span> noexcept-&gt;size_type;</div>
<div class="line"><a name="l00047"></a><span class="lineno">   47</span>&#160;    };</div>
<div class="line"><a name="l00048"></a><span class="lineno">   48</span>&#160; </div>
<div class="line"><a name="l00049"></a><span class="lineno">   49</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00050"></a><span class="lineno">   50</span>&#160;    ring_queue_lockfree&lt;_Ty, _Sty&gt;::ring_queue_lockfree(<span class="keywordtype">size_t</span> sz)</div>
<div class="line"><a name="l00051"></a><span class="lineno">   51</span>&#160;        : m_bufferPtr(new value_type[sz + 1])</div>
<div class="line"><a name="l00052"></a><span class="lineno">   52</span>&#160;        , m_bufferSize(static_cast&lt;size_type&gt;(sz + 1))</div>
<div class="line"><a name="l00053"></a><span class="lineno">   53</span>&#160;        , m_writeIndex(0)</div>
<div class="line"><a name="l00054"></a><span class="lineno">   54</span>&#160;        , m_readIndex(0)</div>
<div class="line"><a name="l00055"></a><span class="lineno">   55</span>&#160;        , m_maximumReadIndex(0)</div>
<div class="line"><a name="l00056"></a><span class="lineno">   56</span>&#160;    #ifdef _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</div>
<div class="line"><a name="l00057"></a><span class="lineno">   57</span>&#160;        , m_count(0)</div>
<div class="line"><a name="l00058"></a><span class="lineno">   58</span>&#160;    #endif</div>
<div class="line"><a name="l00059"></a><span class="lineno">   59</span>&#160;    {</div>
<div class="line"><a name="l00060"></a><span class="lineno">   60</span>&#160;        assert(sz &lt; (std::numeric_limits&lt;size_type&gt;::max)());</div>
<div class="line"><a name="l00061"></a><span class="lineno">   61</span>&#160;    }</div>
<div class="line"><a name="l00062"></a><span class="lineno">   62</span>&#160; </div>
<div class="line"><a name="l00063"></a><span class="lineno">   63</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00064"></a><span class="lineno">   64</span>&#160;    <span class="keyword">auto</span> ring_queue_lockfree&lt;_Ty, _Sty&gt;::countToIndex(size_type a_count) <span class="keyword">const</span> noexcept-&gt;size_type</div>
<div class="line"><a name="l00065"></a><span class="lineno">   65</span>&#160;    {</div>
<div class="line"><a name="l00066"></a><span class="lineno">   66</span>&#160;        <span class="comment">//return (a_count % m_bufferSize);</span></div>
<div class="line"><a name="l00067"></a><span class="lineno">   67</span>&#160;        <span class="keywordflow">return</span> a_count;</div>
<div class="line"><a name="l00068"></a><span class="lineno">   68</span>&#160;    }</div>
<div class="line"><a name="l00069"></a><span class="lineno">   69</span>&#160; </div>
<div class="line"><a name="l00070"></a><span class="lineno">   70</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00071"></a><span class="lineno">   71</span>&#160;    <span class="keyword">auto</span> ring_queue_lockfree&lt;_Ty, _Sty&gt;::nextIndex(size_type a_count) <span class="keyword">const</span> noexcept-&gt;size_type</div>
<div class="line"><a name="l00072"></a><span class="lineno">   72</span>&#160;    {</div>
<div class="line"><a name="l00073"></a><span class="lineno">   73</span>&#160;        <span class="comment">//return static_cast&lt;size_type&gt;((a_count + 1));</span></div>
<div class="line"><a name="l00074"></a><span class="lineno">   74</span>&#160;        <span class="keywordflow">return</span> <span class="keyword">static_cast&lt;</span>size_type<span class="keyword">&gt;</span>((a_count + 1) % m_bufferSize);</div>
<div class="line"><a name="l00075"></a><span class="lineno">   75</span>&#160;    }</div>
<div class="line"><a name="l00076"></a><span class="lineno">   76</span>&#160; </div>
<div class="line"><a name="l00077"></a><span class="lineno">   77</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00078"></a><span class="lineno">   78</span>&#160;    <span class="keyword">auto</span> ring_queue_lockfree&lt;_Ty, _Sty&gt;::size() const noexcept-&gt;size_type</div>
<div class="line"><a name="l00079"></a><span class="lineno">   79</span>&#160;    {</div>
<div class="line"><a name="l00080"></a><span class="lineno">   80</span>&#160;<span class="preprocessor">    #ifdef _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00081"></a><span class="lineno">   81</span>&#160;        <span class="keywordflow">return</span> m_count.load();</div>
<div class="line"><a name="l00082"></a><span class="lineno">   82</span>&#160;<span class="preprocessor">    #else</span></div>
<div class="line"><a name="l00083"></a><span class="lineno">   83</span>&#160;        <span class="keyword">auto</span> currentWriteIndex = m_maximumReadIndex.load(std::memory_order_acquire);</div>
<div class="line"><a name="l00084"></a><span class="lineno">   84</span>&#160;        currentWriteIndex = countToIndex(currentWriteIndex);</div>
<div class="line"><a name="l00085"></a><span class="lineno">   85</span>&#160; </div>
<div class="line"><a name="l00086"></a><span class="lineno">   86</span>&#160;        <span class="keyword">auto</span> currentReadIndex = m_readIndex.load(std::memory_order_acquire);</div>
<div class="line"><a name="l00087"></a><span class="lineno">   87</span>&#160;        currentReadIndex = countToIndex(currentReadIndex);</div>
<div class="line"><a name="l00088"></a><span class="lineno">   88</span>&#160; </div>
<div class="line"><a name="l00089"></a><span class="lineno">   89</span>&#160;        <span class="keywordflow">if</span> (currentWriteIndex &gt;= currentReadIndex)</div>
<div class="line"><a name="l00090"></a><span class="lineno">   90</span>&#160;            <span class="keywordflow">return</span> (currentWriteIndex - currentReadIndex);</div>
<div class="line"><a name="l00091"></a><span class="lineno">   91</span>&#160;        <span class="keywordflow">else</span></div>
<div class="line"><a name="l00092"></a><span class="lineno">   92</span>&#160;            <span class="keywordflow">return</span> (m_bufferSize + currentWriteIndex - currentReadIndex);</div>
<div class="line"><a name="l00093"></a><span class="lineno">   93</span>&#160;<span class="preprocessor">    #endif // _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00094"></a><span class="lineno">   94</span>&#160;    }</div>
<div class="line"><a name="l00095"></a><span class="lineno">   95</span>&#160; </div>
<div class="line"><a name="l00096"></a><span class="lineno">   96</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00097"></a><span class="lineno">   97</span>&#160;    <span class="keyword">auto</span> ring_queue_lockfree&lt;_Ty, _Sty&gt;::capacity() const noexcept-&gt;size_type</div>
<div class="line"><a name="l00098"></a><span class="lineno">   98</span>&#160;    {</div>
<div class="line"><a name="l00099"></a><span class="lineno">   99</span>&#160;        <span class="keywordflow">return</span> m_bufferSize - 1;</div>
<div class="line"><a name="l00100"></a><span class="lineno">  100</span>&#160;    }</div>
<div class="line"><a name="l00101"></a><span class="lineno">  101</span>&#160; </div>
<div class="line"><a name="l00102"></a><span class="lineno">  102</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00103"></a><span class="lineno">  103</span>&#160;    <span class="keywordtype">bool</span> ring_queue_lockfree&lt;_Ty, _Sty&gt;::empty() const noexcept</div>
<div class="line"><a name="l00104"></a><span class="lineno">  104</span>&#160;    {</div>
<div class="line"><a name="l00105"></a><span class="lineno">  105</span>&#160;<span class="preprocessor">    #ifdef _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00106"></a><span class="lineno">  106</span>&#160;        <span class="keywordflow">return</span> m_count.load() == 0;</div>
<div class="line"><a name="l00107"></a><span class="lineno">  107</span>&#160;<span class="preprocessor">    #else</span></div>
<div class="line"><a name="l00108"></a><span class="lineno">  108</span>&#160;        <span class="keyword">auto</span> currentWriteIndex = m_maximumReadIndex.load(std::memory_order_acquire);</div>
<div class="line"><a name="l00109"></a><span class="lineno">  109</span>&#160;        <span class="keyword">auto</span> currentReadIndex = m_readIndex.load(std::memory_order_acquire);</div>
<div class="line"><a name="l00110"></a><span class="lineno">  110</span>&#160;        <span class="keywordflow">return</span> countToIndex(currentWriteIndex) == countToIndex(currentReadIndex);</div>
<div class="line"><a name="l00111"></a><span class="lineno">  111</span>&#160;<span class="preprocessor">    #endif // _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00112"></a><span class="lineno">  112</span>&#160;    }</div>
<div class="line"><a name="l00113"></a><span class="lineno">  113</span>&#160; </div>
<div class="line"><a name="l00114"></a><span class="lineno">  114</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00115"></a><span class="lineno">  115</span>&#160;    <span class="keywordtype">bool</span> ring_queue_lockfree&lt;_Ty, _Sty&gt;::full() const noexcept</div>
<div class="line"><a name="l00116"></a><span class="lineno">  116</span>&#160;    {</div>
<div class="line"><a name="l00117"></a><span class="lineno">  117</span>&#160;<span class="preprocessor">    #ifdef _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00118"></a><span class="lineno">  118</span>&#160;        <span class="keywordflow">return</span> (m_count.load() == (m_bufferSize - 1));</div>
<div class="line"><a name="l00119"></a><span class="lineno">  119</span>&#160;<span class="preprocessor">    #else</span></div>
<div class="line"><a name="l00120"></a><span class="lineno">  120</span>&#160;        <span class="keyword">auto</span> currentWriteIndex = m_writeIndex.load(std::memory_order_acquire);</div>
<div class="line"><a name="l00121"></a><span class="lineno">  121</span>&#160;        <span class="keyword">auto</span> currentReadIndex = m_readIndex.load(std::memory_order_acquire);</div>
<div class="line"><a name="l00122"></a><span class="lineno">  122</span>&#160;        <span class="keywordflow">return</span> countToIndex(nextIndex(currentWriteIndex)) == countToIndex(currentReadIndex);</div>
<div class="line"><a name="l00123"></a><span class="lineno">  123</span>&#160;<span class="preprocessor">    #endif // _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00124"></a><span class="lineno">  124</span>&#160;    }</div>
<div class="line"><a name="l00125"></a><span class="lineno">  125</span>&#160; </div>
<div class="line"><a name="l00126"></a><span class="lineno">  126</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00127"></a><span class="lineno">  127</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> U&gt;</div>
<div class="line"><a name="l00128"></a><span class="lineno">  128</span>&#160;    <span class="keywordtype">bool</span> ring_queue_lockfree&lt;_Ty, _Sty&gt;::try_push(U&amp;&amp; value) noexcept(std::is_nothrow_move_constructible_v&lt;U&gt;)</div>
<div class="line"><a name="l00129"></a><span class="lineno">  129</span>&#160;    {</div>
<div class="line"><a name="l00130"></a><span class="lineno">  130</span>&#160;        <span class="keyword">auto</span> currentWriteIndex = m_writeIndex.load(std::memory_order_acquire);</div>
<div class="line"><a name="l00131"></a><span class="lineno">  131</span>&#160; </div>
<div class="line"><a name="l00132"></a><span class="lineno">  132</span>&#160;        <span class="keywordflow">do</span></div>
<div class="line"><a name="l00133"></a><span class="lineno">  133</span>&#160;        {</div>
<div class="line"><a name="l00134"></a><span class="lineno">  134</span>&#160;            <span class="keywordflow">if</span> (countToIndex(nextIndex(currentWriteIndex)) == countToIndex(m_readIndex.load(std::memory_order_acquire)))</div>
<div class="line"><a name="l00135"></a><span class="lineno">  135</span>&#160;            {</div>
<div class="line"><a name="l00136"></a><span class="lineno">  136</span>&#160;                <span class="comment">// the queue is full</span></div>
<div class="line"><a name="l00137"></a><span class="lineno">  137</span>&#160;                <span class="keywordflow">return</span> <span class="keyword">false</span>;</div>
<div class="line"><a name="l00138"></a><span class="lineno">  138</span>&#160;            }</div>
<div class="line"><a name="l00139"></a><span class="lineno">  139</span>&#160; </div>
<div class="line"><a name="l00140"></a><span class="lineno">  140</span>&#160;            <span class="comment">// There is more than one producer. Keep looping till this thread is able </span></div>
<div class="line"><a name="l00141"></a><span class="lineno">  141</span>&#160;            <span class="comment">// to allocate space for current piece of data</span></div>
<div class="line"><a name="l00142"></a><span class="lineno">  142</span>&#160;            <span class="comment">//</span></div>
<div class="line"><a name="l00143"></a><span class="lineno">  143</span>&#160;            <span class="comment">// using compare_exchange_strong because it isn&#39;t allowed to fail spuriously</span></div>
<div class="line"><a name="l00144"></a><span class="lineno">  144</span>&#160;            <span class="comment">// When the compare_exchange operation is in a loop the weak version</span></div>
<div class="line"><a name="l00145"></a><span class="lineno">  145</span>&#160;            <span class="comment">// will yield better performance on some platforms, but here we&#39;d have to</span></div>
<div class="line"><a name="l00146"></a><span class="lineno">  146</span>&#160;            <span class="comment">// load m_writeIndex all over again</span></div>
<div class="line"><a name="l00147"></a><span class="lineno">  147</span>&#160;        } <span class="keywordflow">while</span> (!m_writeIndex.compare_exchange_strong(currentWriteIndex, nextIndex(currentWriteIndex), std::memory_order_acq_rel));</div>
<div class="line"><a name="l00148"></a><span class="lineno">  148</span>&#160; </div>
<div class="line"><a name="l00149"></a><span class="lineno">  149</span>&#160;        <span class="comment">// Just made sure this index is reserved for this thread.</span></div>
<div class="line"><a name="l00150"></a><span class="lineno">  150</span>&#160;        m_bufferPtr[countToIndex(currentWriteIndex)] = std::move(value);</div>
<div class="line"><a name="l00151"></a><span class="lineno">  151</span>&#160; </div>
<div class="line"><a name="l00152"></a><span class="lineno">  152</span>&#160;        <span class="comment">// update the maximum read index after saving the piece of data. It can&#39;t</span></div>
<div class="line"><a name="l00153"></a><span class="lineno">  153</span>&#160;        <span class="comment">// fail if there is only one thread inserting in the queue. It might fail </span></div>
<div class="line"><a name="l00154"></a><span class="lineno">  154</span>&#160;        <span class="comment">// if there is more than 1 producer thread because this operation has to</span></div>
<div class="line"><a name="l00155"></a><span class="lineno">  155</span>&#160;        <span class="comment">// be done in the same order as the previous CAS</span></div>
<div class="line"><a name="l00156"></a><span class="lineno">  156</span>&#160;        <span class="comment">//</span></div>
<div class="line"><a name="l00157"></a><span class="lineno">  157</span>&#160;        <span class="comment">// using compare_exchange_weak because they are allowed to fail spuriously</span></div>
<div class="line"><a name="l00158"></a><span class="lineno">  158</span>&#160;        <span class="comment">// (act as if *this != expected, even if they are equal), but when the</span></div>
<div class="line"><a name="l00159"></a><span class="lineno">  159</span>&#160;        <span class="comment">// compare_exchange operation is in a loop the weak version will yield</span></div>
<div class="line"><a name="l00160"></a><span class="lineno">  160</span>&#160;        <span class="comment">// better performance on some platforms.</span></div>
<div class="line"><a name="l00161"></a><span class="lineno">  161</span>&#160;        <span class="keyword">auto</span> savedWriteIndex = currentWriteIndex;</div>
<div class="line"><a name="l00162"></a><span class="lineno">  162</span>&#160;        <span class="keywordflow">while</span> (!m_maximumReadIndex.compare_exchange_weak(currentWriteIndex, nextIndex(currentWriteIndex), std::memory_order_acq_rel))</div>
<div class="line"><a name="l00163"></a><span class="lineno">  163</span>&#160;        {</div>
<div class="line"><a name="l00164"></a><span class="lineno">  164</span>&#160;            currentWriteIndex = savedWriteIndex;</div>
<div class="line"><a name="l00165"></a><span class="lineno">  165</span>&#160;            <span class="comment">// this is a good place to yield the thread in case there are more</span></div>
<div class="line"><a name="l00166"></a><span class="lineno">  166</span>&#160;            <span class="comment">// software threads than hardware processors and you have more</span></div>
<div class="line"><a name="l00167"></a><span class="lineno">  167</span>&#160;            <span class="comment">// than 1 producer thread</span></div>
<div class="line"><a name="l00168"></a><span class="lineno">  168</span>&#160;            <span class="comment">// have a look at sched_yield (POSIX.1b)</span></div>
<div class="line"><a name="l00169"></a><span class="lineno">  169</span>&#160;            std::this_thread::yield();</div>
<div class="line"><a name="l00170"></a><span class="lineno">  170</span>&#160;        }</div>
<div class="line"><a name="l00171"></a><span class="lineno">  171</span>&#160; </div>
<div class="line"><a name="l00172"></a><span class="lineno">  172</span>&#160;        <span class="comment">// The value was successfully inserted into the queue</span></div>
<div class="line"><a name="l00173"></a><span class="lineno">  173</span>&#160;<span class="preprocessor">    #ifdef _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00174"></a><span class="lineno">  174</span>&#160;        m_count.fetch_add(1);</div>
<div class="line"><a name="l00175"></a><span class="lineno">  175</span>&#160;<span class="preprocessor">    #endif</span></div>
<div class="line"><a name="l00176"></a><span class="lineno">  176</span>&#160;        <span class="keywordflow">return</span> <span class="keyword">true</span>;</div>
<div class="line"><a name="l00177"></a><span class="lineno">  177</span>&#160;    }</div>
<div class="line"><a name="l00178"></a><span class="lineno">  178</span>&#160; </div>
<div class="line"><a name="l00179"></a><span class="lineno">  179</span>&#160;    <span class="keyword">template</span>&lt;<span class="keyword">class</span> _Ty, <span class="keyword">class</span> _Sty&gt;</div>
<div class="line"><a name="l00180"></a><span class="lineno">  180</span>&#160;    <span class="keywordtype">bool</span> ring_queue_lockfree&lt;_Ty, _Sty&gt;::try_pop(value_type&amp; value) noexcept(std::is_nothrow_move_constructible_v&lt;value_type&gt;)</div>
<div class="line"><a name="l00181"></a><span class="lineno">  181</span>&#160;    {</div>
<div class="line"><a name="l00182"></a><span class="lineno">  182</span>&#160;        <span class="keyword">auto</span> currentReadIndex = m_readIndex.load(std::memory_order_acquire);</div>
<div class="line"><a name="l00183"></a><span class="lineno">  183</span>&#160; </div>
<div class="line"><a name="l00184"></a><span class="lineno">  184</span>&#160;        <span class="keywordflow">for</span> (;;)</div>
<div class="line"><a name="l00185"></a><span class="lineno">  185</span>&#160;        {</div>
<div class="line"><a name="l00186"></a><span class="lineno">  186</span>&#160;            <span class="keyword">auto</span> idx = countToIndex(currentReadIndex);</div>
<div class="line"><a name="l00187"></a><span class="lineno">  187</span>&#160; </div>
<div class="line"><a name="l00188"></a><span class="lineno">  188</span>&#160;            <span class="comment">// to ensure thread-safety when there is more than 1 producer </span></div>
<div class="line"><a name="l00189"></a><span class="lineno">  189</span>&#160;            <span class="comment">// thread a second index is defined (m_maximumReadIndex)</span></div>
<div class="line"><a name="l00190"></a><span class="lineno">  190</span>&#160;            <span class="keywordflow">if</span> (idx == countToIndex(m_maximumReadIndex.load(std::memory_order_acquire)))</div>
<div class="line"><a name="l00191"></a><span class="lineno">  191</span>&#160;            {</div>
<div class="line"><a name="l00192"></a><span class="lineno">  192</span>&#160;                <span class="comment">// the queue is empty or</span></div>
<div class="line"><a name="l00193"></a><span class="lineno">  193</span>&#160;                <span class="comment">// a producer thread has allocate space in the queue but is </span></div>
<div class="line"><a name="l00194"></a><span class="lineno">  194</span>&#160;                <span class="comment">// waiting to commit the data into it</span></div>
<div class="line"><a name="l00195"></a><span class="lineno">  195</span>&#160;                <span class="keywordflow">return</span> <span class="keyword">false</span>;</div>
<div class="line"><a name="l00196"></a><span class="lineno">  196</span>&#160;            }</div>
<div class="line"><a name="l00197"></a><span class="lineno">  197</span>&#160; </div>
<div class="line"><a name="l00198"></a><span class="lineno">  198</span>&#160;            <span class="comment">// retrieve the data from the queue</span></div>
<div class="line"><a name="l00199"></a><span class="lineno">  199</span>&#160;            value = m_bufferPtr[idx];       <span class="comment">//但是，这里的方法不适合。如果只支持移动怎么办？</span></div>
<div class="line"><a name="l00200"></a><span class="lineno">  200</span>&#160; </div>
<div class="line"><a name="l00201"></a><span class="lineno">  201</span>&#160;            <span class="comment">// try to perfrom now the CAS operation on the read index. If we succeed</span></div>
<div class="line"><a name="l00202"></a><span class="lineno">  202</span>&#160;            <span class="comment">// a_data already contains what m_readIndex pointed to before we </span></div>
<div class="line"><a name="l00203"></a><span class="lineno">  203</span>&#160;            <span class="comment">// increased it</span></div>
<div class="line"><a name="l00204"></a><span class="lineno">  204</span>&#160;            <span class="keywordflow">if</span> (m_readIndex.compare_exchange_strong(currentReadIndex, nextIndex(currentReadIndex), std::memory_order_acq_rel))</div>
<div class="line"><a name="l00205"></a><span class="lineno">  205</span>&#160;            {</div>
<div class="line"><a name="l00206"></a><span class="lineno">  206</span>&#160;                <span class="comment">// got here. The value was retrieved from the queue. Note that the</span></div>
<div class="line"><a name="l00207"></a><span class="lineno">  207</span>&#160;                <span class="comment">// data inside the m_queue array is not deleted nor reseted</span></div>
<div class="line"><a name="l00208"></a><span class="lineno">  208</span>&#160;<span class="preprocessor">    #ifdef _WITH_LOCK_FREE_Q_KEEP_REAL_SIZE</span></div>
<div class="line"><a name="l00209"></a><span class="lineno">  209</span>&#160;                m_count.fetch_sub(1);</div>
<div class="line"><a name="l00210"></a><span class="lineno">  210</span>&#160;<span class="preprocessor">    #endif</span></div>
<div class="line"><a name="l00211"></a><span class="lineno">  211</span>&#160;                <span class="keywordflow">return</span> <span class="keyword">true</span>;</div>
<div class="line"><a name="l00212"></a><span class="lineno">  212</span>&#160;            }</div>
<div class="line"><a name="l00213"></a><span class="lineno">  213</span>&#160; </div>
<div class="line"><a name="l00214"></a><span class="lineno">  214</span>&#160;            <span class="comment">// it failed retrieving the element off the queue. Someone else must</span></div>
<div class="line"><a name="l00215"></a><span class="lineno">  215</span>&#160;            <span class="comment">// have read the element stored at countToIndex(currentReadIndex)</span></div>
<div class="line"><a name="l00216"></a><span class="lineno">  216</span>&#160;            <span class="comment">// before we could perform the CAS operation        </span></div>
<div class="line"><a name="l00217"></a><span class="lineno">  217</span>&#160;        }   <span class="comment">// keep looping to try again!</span></div>
<div class="line"><a name="l00218"></a><span class="lineno">  218</span>&#160;    }</div>
<div class="line"><a name="l00219"></a><span class="lineno">  219</span>&#160;}</div>
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